Optimizing the formulation of coal gangue planting substrate using wastes: The sustainability of coal mine ecological restoration

With the development of China's coal industry, coal gangue has become one of the most extensive solid wastes. The collection of soil from other places for land reclamation has become a commonly used technology for ecological restoration of coal-mining areas in China. The purpose of this study was to use coal gangue as a new type of planting substrate component, with the goal of replacing soil to solve the increasing shortage of land resources. The formulation of the substrate was optimized using an orthogonal experiment with four factors: coal gangue (g)-to-soil (g) ratio (1000:0, 750:250, 500:500), maize straw content (0, 25, and 50 g kg(-1)), fly ash content (0, 75, and 150 g kg(-1)), and water retention agent content (0, 1, and 2 g kg(-1)).The results showed that coal gangue significantly improved the chemical properties (organic matter, total N, total P, available N, available P and available K) of the substrate (P < .01) at a dosage of 1000 g and increased alfalfa growth and improved the soil nutrient ratio at a dosage of 500 g. In addition, maize straw significantly affected the physical properties (bulk density, total porosity, capillary porosity, soil moisture content and permeability coefficient) of the substrate (P < .01) at 50 g and was most beneficial to plant growth (P < .01) at 25 g. Fly ash caused certain changes in the physical and chemical properties of the substrate and thus affected plant growth; the water retention agent showed a lesser effect. After the orthogonal experiments, we used a model to comprehensively analyze the effects of the four factors on 21 parameters to determine the optimum ratios of the components, which were as follows: a coal gangue-to-soil ratio of 1:1 (500 g:500 g), a maize straw content of 50 g kg(-1), a fly ash content of 37 g kg(-1), and a water retention agent content of 1 g kg(-1). The findings of this study will provide a theoretical and technical basis for the ecological restoration of coal mining areas and the resource utilization of coal gangue.